Floating driving circuit

ABSTRACT

A floating driving circuit according to the present invention comprises an input circuit to receive an input signal. A latch circuit receives a trigger signal for generating a latch signal. The latch signal is used to turn on/off a switch. A coupling capacitor is connected between the input circuit and the latch circuit to generate the trigger signal in response to the input signal. A diode is connected from a voltage source to a floating supply terminal of the latch circuit for charging a capacitor. The capacitor is coupled between the floating supply terminal and a floating ground terminal of the latch circuit to provide a supply voltage to the latch circuit. The latch circuit is controlled by the input signal via the coupling capacitor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driving circuit for a switch, andmore particularly to a floating driving circuit for driving the switch.

2. Description of the Prior Art

A variety of power converters and motor drivers utilize bridge circuitsto control power delivery from a power source to a load. The bridgecircuit generally has a high-side switch coupled to the power source anda low-side switch coupled to a ground reference. A common node of thehigh-side switch and the low-side switch is coupled to the load.High-side and low-side switches are generally implemented withtransistors. When high-side and low-side switches are controlled toalternately conduct, a voltage level at the common node swings betweenthe power source and the ground reference. Therefore, the voltage levelof the common node shifts to the power source when the high-side switchis turned on. In order to fully turn on the high-side switch to achievelow impedance, a gate driving voltage higher than the power source isrequired. Therefore, gate-to-source of the high-side switch must befloated with respect to the ground reference.

FIG. 1 shows a circuit diagram of a conventional bridge circuit having abootstrap capacitor 44 and a charge-pump diode 40 to create a floatingvoltage V_(CC) for driving a gate of a high-side switch 10. Thehigh-side switch 10 receives an input voltage V_(IN). When a controllingtransistor 45 is turned on, the gate of the high-side switch 10 isconnected to the ground reference via a diode 42. This will turn off thehigh-side switch 10. The controlling transistor 45 is controlled by aninput signal S_(IN) via an inverter 43. Once the high-side switch 10 isturned off and a low-side switch 20 is turned on, the bootstrapcapacitor 44 will be charged up to the floating voltage V_(CC) by a biasvoltage V_(B) via the charge-pump diode 40. The low-side switch 20 iscoupled to the ground reference. Turning off the controlling transistor45 will propagate the floating voltage V_(CC) via a transistor 41 to thegate of the high-side switch 10. This turns on the high-side switch 10.A resistor 46 is coupled between the charge-pump diode 40 and thetransistor 41.

One drawback of this circuit is its high switching losses inhigh-voltage applications. The controlling transistor 45 requires ahigh-voltage manufacturing process to be suitable for high-voltageapplications (such as 200 volts or more). However, the parasiticcapacitor of a high-voltage transistor is generally large, which willincrease a rising time of a switching signal and therefore slow down theswitching operation of the high-voltage transistor. This further causeshigh switching losses of the high-side switch 10. Therefore, this bridgecircuit is inadequate for high-voltage and high-speed applications.

Many recently developed bridge circuit designs include methods ofgenerating a suitable gate voltage for the high-side switch. Somewell-known inventions include U.S. Pat. No. 5,381,044 (Zisa, Belluso,Paparo), U.S. Pat. No. 5,638,025 (Johnson), and U.S. Pat. No. 5,672,992(Nadd). These bridge circuits share the same drawbacks as the circuitshown in FIG. 1. The controlling transistors of the aforementionedinventions cause high switching losses in high-voltage applications.

To overcome some of these disadvantages, a bridge circuit utilizing aboost converter technique has been introduced in U.S. Pat. No. 6,344,959(Milazzo). However, this technique uses a voltage doubling circuit thatrequires an additional switching element as well as other circuitries,thereby increasing the cost and complexity of the driving circuit. Otherprior arts such as U.S. Pat. No. 6,781,422 (Yang) and U.S. Pat. No.6,836,173 (Yang) disclosed the high-side transistor driver forhigh-speed applications, but the higher power consumption is still theissue to be concerned.

An objective of the present invention is to overcome the drawbacks ofprior arts. Another objective is to eliminate the need of high voltagecontrolling transistor (such as the controlling transistor 45) forproviding a high efficiency driving circuit in high-voltage andhigh-speed applications.

SUMMARY OF THE INVENTION

A floating driving circuit according to the present invention comprisesan input circuit to receive an input signal. A latch circuit is coupledto receive a trigger signal for generating a latch signal. The latchsignal is used to drive a switch. A coupling capacitor is connectedbetween the input circuit and the latch circuit to generate the triggersignal in response to the input signal. A diode is connected from avoltage source to a floating supply terminal of the latch circuit forcharging a capacitor. The capacitor is coupled between the floatingsupply terminal and a floating ground terminal of the latch circuit toprovide a supply voltage to the latch circuit. The latch circuit iscontrolled by the input signal via the coupling capacitor. A fallingedge and a rising edge of the input signal determine the state of thelatch circuit. The latch circuit will retain the state to turn on/offthe switch. Therefore, no high voltage controlling transistor is needed.

The floating driving circuit introduces a method to drive the switch inhigh-voltage and high-speed applications. Moreover, the floating drivingcircuit provides a high efficiency switching operation for power saving.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention, and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 shows a circuit diagram of a conventional bridge circuit.

FIG. 2 shows a circuit diagram of a preferred embodiment of a floatingdriving circuit according to the present invention.

FIG. 3 shows a circuit diagram of an embodiment of a latch circuitaccording to the present invention.

FIG. 4 shows a circuit diagram of another embodiment of the latchcircuit according to the present invention.

FIG. 5 shows a circuit diagram of another embodiment of the latchcircuit according to the present invention.

FIG. 6 shows a circuit diagram of an embodiment of a differentialfloating driving circuit according to the present invention.

FIG. 7 shows a circuit diagram of an embodiment of a RS latch circuitaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a circuit diagram of a floating driving circuit accordingto an embodiment of the present invention. It comprises an input circuit60 having an input terminal for receiving an input signal S_(IN). Theinput circuit 60 operates as an inverter. An input terminal R/S of alatch circuit 100 receives a trigger signal. The latch circuit 100further has an output terminal Q for generating a driving signal todrive a high-side switch 10. The high-side switch 10 receives an inputvoltage V_(IN). Wherein the driving signal is a latch signal and thehigh-side switch 10 is implemented with a transistor. A low-side switch20 is coupled between the high-side switch 10 and a ground reference.

A coupling capacitor 50 is coupled between an output terminal of theinput circuit 60 and the input terminal R/S of the latch circuit 100 togenerate the trigger signal in response to the input signal S_(IN). Thelatch circuit 100 will change the state of the latch signal in responseto the change of the trigger signal. That is, the state of the latchsignal will change in response to the change of the input signal S_(IN).A falling edge and a rising edge of the input signal S_(IN) determinethe state of the latch signal. The latch circuit 100 will retain thestate to turn on/off the high-side switch 10. Therefore, no high-voltagecontrolling transistor is needed.

An isolation barrier or a high voltage would be produced between theinput circuit 60 and the latch circuit 100. Therefore, the couplingcapacitor 50 is required to be a high voltage capacitor to sustain thehigh voltage across the barrier. The latch circuit 100 includes a firstterminal (floating supply terminal) VP and a second terminal (floatingground terminal) VN. The floating supply terminal VP and the floatingground terminal VN are used for receiving a supply voltage. The floatingground terminal VN is further connected to the high-side switch 10. Adiode 35 is connected between a voltage source V_(D) and the floatingsupply terminal VP. A capacitor 30 is coupled between the floatingsupply terminal VP and the floating ground terminal VN to store theenergy for the latch circuit 100. The voltage source V_(D) will chargethe capacitor 30 to provide the supply voltage to the latch circuit 100when the high-side switch 10 is turned off.

FIG. 3 is an embodiment of the latch circuit 100. The latch circuit 100operates as a switch driving circuit including a positive feedback. Itcomprises a buffer circuit 180, a first inverter circuit 185, a secondinverter circuit 170, a latch transistor 150, a first resistive device120 and a second resistive device 125. Resistive devices 120 and 125 canbe implemented by resistors or transistors or current sources. An inputterminal of the buffer circuit 180 is coupled to the input terminal R/Sof the latch circuit 100 to receive the trigger signal. The firstinverter circuit 185 has an input terminal connected to an outputterminal of the buffer circuit 180 for generating the latch signal at anoutput terminal of the first inverter circuit 185. The output terminalof the first inverter circuit 185 is coupled to the output terminal Q ofthe latch circuit 100.

The first resistive device 120 is connected between the floating supplyterminal VP and the input terminal R/S of the latch circuit 100. Thesecond resistive device 125 is connected in series with the latchtransistor 150. The second resistive device 125 is connected to theinput terminal R/S of the latch circuit 100. The latch transistor 150 isconnected to the floating ground terminal VN. An input terminal of thesecond inverter circuit 170 is coupled to the output terminal of thebuffer circuit 180. An output terminal of the second inverter circuit170 is coupled to the latch transistor 150 to control the latchtransistor 150. The buffer circuit 180, the second inverter circuit 170,the latch transistor 150 and the second resistive device 125 form apositive feedback loop for the latch function.

FIG. 4 shows another embodiment of the latch circuit 100. The currentsources 110 and 115 operate as resistive devices 120 and 125 shown inFIG. 3. FIG. 5 shows another embodiment of the latch circuit 100. Atransistor 160 and a third resistive device 165 operate as the secondinverter circuit 170 shown in FIG. 3. The transistor 160 and the thirdresistive device 165 are connected in series. The third resistive device165 is further coupled to the floating supply terminal VP.

In order to achieve better noise immunity, a differential floatingdriving circuit shown in FIG. 6 is developed according to the presentinvention. It comprises an input circuit 65 including a buffer 66 and aninverter 67 to receive the input signal StN. An input terminal of thebuffer 66 and an input terminal of the inverter 67 are coupled togetherto receive the input signal S_(IN). A floating differential circuit 90comprises a first comparator 70, a second comparator 80 and a resistivedevice 95. The floating differential circuit 90 receives differentialtrigger signals for generating a set signal and a reset signal. Afloating latch circuit 200 has a set terminal S and a reset terminal Rto receive the set signal and the reset signal respectively forgenerating a latch signal at an output terminal Q of the floating latchcircuit 200. The capacitor 30 is coupled to the floating latch circuit200.

The floating latch circuit 200 has a positive feedback to change thelatch state of the latch signal in response to the change of thedifferential trigger signals. The latch signal is used to control thehigh-side switch 10. Coupling capacitors 56 and 57 are coupled betweenthe input circuit 65 and the floating differential circuit 90 togenerate the differential trigger signals in response to the inputsignal S_(IN). The coupling capacitor 56 is coupled between an outputterminal of the buffer 66 of the input circuit 65 and an input terminalof the floating differential circuit 90. The coupling capacitor 57 iscoupled between an output terminal of the inverter 67 of the inputcircuit 65 and another input terminal of the floating differentialcircuit 90. Because the differential trigger signals are generated indifferential mode, a common mode noise cannot interrupt the operation ofthe differential floating driving circuit.

An output terminal of the first comparator 70, which is connected to thereset terminal R of the floating latch circuit 200, generates the resetsignal. An output terminal of the second comparator 80, which isconnected to the set terminal S of the floating latch circuit 200,generates the set signal. The resistive device 95 is coupled betweennegative input terminals of the comparator 70 and 80 to provideimpedance for the termination. Negative input terminals of thecomparator 70 and 80 are connected to the input terminals of thefloating differential circuit 90. A positive input terminal of the firstcomparator 70 is connected to the negative input terminal of the secondcomparator 80 via a first threshold 75. A positive input terminal of thesecond comparator 80 is connected to the negative input terminal of thefirst comparator 70 via a second threshold 85. Therefore, the resetsignal and the set signal can only be generated when the differentialtrigger signals are generated in differential mode. Besides, theamplitude of differential trigger signals must be higher than the firstthreshold or the second threshold for changing the state of the latchsignal.

FIG. 7 shows the floating latch circuit 200. It is a RS latch circuitincluding inverters 210, 215 and NAND gates 230, 235. An input terminalof the inverter 210 is connected to the set terminal S. An inputterminal of the inverter 215 is connected to the reset terminal R. Anoutput terminal of the inverter 210 is connected to an input terminal ofthe NAND gate 230. An output terminal of the inverter 215 is connectedto an input terminal of the NAND gate 235. An output terminal of theNAND gate 230 generates the latch signal at the output terminal Q of thefloating latch circuit 200. The output terminal of the NAND gate 230 isfurther connected to another input terminal of the NAND gate 235. Anoutput terminal of the NAND gate 235 is connected to another inputterminal of the NAND gate 230 to form the positive feedback for thelatch operation.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims or their equivalents.

1. A floating driving circuit, consisting of: an input circuit, forreceiving an input signal; a latch circuit, only having a single inputterminal for receiving a single trigger signal for generating a latchsignal to turn on/off a switch, said latch circuit having a firstterminal and a second terminal for receiving a supply voltage, whereinsaid second terminal is connected to said switch; a coupling capacitor,connected between said input circuit and said single input terminal ofsaid latch circuit to generate said single trigger signal for turningon/off said switch in response to said input signal; a diode, coupledfrom a voltage source to said first terminal; and a capacitor, coupledbetween said first terminal and said second terminal for providing saidsupply voltage; wherein said latch circuit changes a state of said latchsignal in response to a change of said single trigger signal.
 2. Thefloating driving circuit as claimed in claim 1, wherein said latchcircuit has a positive feedback.
 3. The floating driving circuit asclaimed in claim 1, wherein said latch circuit comprises: a buffercircuit, having an input terminal coupled to said single input terminalof said latch circuit to receive said single trigger signal; a firstinverter circuit, having an input terminal connected to an outputterminal of said buffer circuit for generating said latch signal; afirst resistive device, connected from said first terminal to saidsingle input terminal of said latch circuit; a second resistive device,connected to said single input terminal of said latch circuit; a latchtransistor, connected in series with said second resistive device,wherein said latch transistor is connected to said second terminal; anda second inverter circuit, having an input terminal coupled to saidoutput terminal of said buffer circuit, wherein an output terminal ofsaid second inverter circuit is coupled to said latch transistor tocontrol said latch transistor.
 4. The floating driving circuit asclaimed in claim 3, wherein said first resistive device and said secondresistive device are implemented by one of resistors or transistors orcurrent sources.
 5. A driving circuit, comprising: an input circuit, forreceiving an input signal; a latch circuit, only having a single inputterminal for receiving a single trigger signal for generating a latchsignal to turn on/off a switch; and a coupling capacitor, coupledbetween said input circuit and said single input terminal of said latchcircuit to generate said single trigger signal for turning on/off saidswitch in response to said input signal; wherein said latch circuitchanges a state of said latch signal in response to a change of saidsingle trigger signal; in which said latch circuit comprises: a buffercircuit, having an input terminal coupled to said single input terminalof said latch circuit to receive said single trigger signal; a firstinverter circuit, having an input terminal connected to an outputterminal of said buffer circuit for generating said latch signal; afirst resistive device, connected from a floating supply terminal ofsaid latch circuit to said single input terminal of said latch circuit;a second resistive device, connected to said single input terminal ofsaid latch circuit; a latch transistor, connected in series with saidsecond resistive device, wherein said latch transistor is connected to afloating ground terminal of said latch circuit, and a second invertercircuit, having an input terminal coupled to said output terminal ofsaid buffer circuit, wherein an output terminal of said second invertercircuit is coupled to said latch transistor to control said latchtransistor.
 6. The driving circuit as claimed in claim 5, wherein saidlatch circuit has a positive feedback.
 7. The driving circuit as claimedin claim 5, wherein said first resistive device and said secondresistive device are implemented by one of resistors or transistors orcurrent sources.
 8. A floating driving circuit for a high-side switch,comprising: an input circuit, for receiving an input signal; a drivingcircuit, only having a single input terminal for receiving a singletrigger signal for generating a driving signal to turn on/off saidhigh-side switch; and a coupling capacitor, coupled between said inputcircuit and said single input terminal of said driving circuit togenerate said single trigger signal for turning on/off said high-sideswitch in response to said input signal; wherein said driving circuitchanges a state of said driving signal in response to a change of saidinput signal; in which said driving circuit comprises: a buffer circuit,having an input terminal coupled to said single input terminal of saiddriving circuit to receive said single trigger signal; a first invertercircuit, having an input terminal connected to an output terminal ofsaid buffer circuit for generating said driving signal; a firstresistive device, connected from a floating supply terminal of saiddriving circuit to said single input terminal of said driving circuit; asecond resistive device, connected to said single input terminal of saiddriving circuit; a latch transistor, connected in series with saidsecond resistive device, wherein said latch transistor is connected to afloating ground terminal of said driving circuit; and a second invertercircuit, having an input terminal coupled to said output terminal ofsaid buffer circuit, wherein an output terminal of said second invertercircuit is coupled to said latch transistor to control said latchtransistor.
 9. The floating driving circuit as claimed in claim 8,wherein said driving circuit has a positive feedback.
 10. The floatingdriving circuit as claimed in claim 8, wherein said first resistivedevice and said second resistive device are implemented by one ofresistors or transistors or current sources.